Britney makes so much money because people are idiots and buy her music and love to read about her in the tabloids. American culture is driving the hype machine that nets her so much money -- Britney herself has little to do with it. I think you're confusing actor/musician with pop culture superstar. For every Britney Spears or Angeline Jolie, there are countless other actors and musicians who are just barely getting by and have to supplement their meager incomes by waiting tables. Visit a college campus and find a musical theater major or a music performance major and talk to them a bit. These guys are not going to make much money when they get out of school, at least not from their performance abilities, because there's not much of a market for them -- and they know it.
I, for example, would love to compose art music for a living, but that just ain't gonna happen because there's no market for it. Not enough people want to listen to the stuff I want to compose for it to be feasible. To make any sort of living in music, I'll either have to break into the film/media market (difficult) or or find a university to teach at (my current long-term goal). Last I checked, no one was offering me a Britney Spears-type deal.
Do you believe that as an architect, you deserve to make more money than firefighters who risk their lives doing their "job"? Do you believe that as an airline pilot ... ? Do you believe that as a business consultant ... ? Do you see what I'm driving at here? All of these occupations are potentially more profitable and less dangerous than firefighting, so why single out musicians? The amount of money someone makes is determined by market forces, not by who is arguably morally superior.